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How to update a JPanel with an image that is refreshed every second

Posted on 2008-06-11
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Last Modified: 2013-12-14
I have a JPanel within a JFrame in a basic swing application. I need to update the panel by redrawing an image that changes every second at the same location / URL (this is from an IP Camera). The application reads the image every second, but does not refresh the panel with the new image. I'm currently trying to use the 'update' method of the JPanel class for this task.
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Question by:jsonburke
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by:objects
ID: 21764925
use a timer that calls repaint() every minute

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by:objects
ID: 21764935
why doesn't the app repaint the image after it reads the image?

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Author Comment

by:jsonburke
ID: 21764980
I may be mistaken, but doesn't the update method call the repaint method? Or, is it vice versa?

Why I'm seeing this behavior is really a part of my question... my *guess* is because the image is at the same location so it is using a version cached in memory somewhere
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by:objects
ID: 21764988
how are you currently updating it?

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by:jsonburke
ID: 21765015
This is the paint method I'm overriding in a separate jar at the moment just to get the update functionality to work as a start.

(Ultimately I will need to do some multi-thread work for the subpanel to display the feed without freezing the app once it connects)
public void paint(Graphics g) {
        // show the number of calls to paint()
        System.out.println(++n);
        // if there is an image and drawing has not finished
        // note: if the image is null drawImage will return true
        if (image == null ||!g.drawImage(image,0,0,getWidth(),getHeight(),this )) {
            // draw the message
            g.drawString(msg,20,getHeight()/2);
            System.out.println(msg);
        }
        
        if (n < 30) {
        try {
            Thread.sleep(1000);
            this.update(this.getGraphics());
        } catch (InterruptedException ex) {
            Logger.getLogger(ImageViewTest.class.getName()).log(Level.SEVERE, null, ex);
        }
        } else {
            System.exit(0);
        }
    }

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Expert Comment

by:objects
ID: 21765021
repaint() will trigger a call to update()
You typically shouldn't be calling update() directly.

http://java.sun.com/products/jfc/tsc/articles/painting/

Sounds like the problem though is with the actual image loading.
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Expert Comment

by:objects
ID: 21765040
       if (n < 30) {
        try {
            Thread.sleep(1000);
            this.update(this.getGraphics());
        } catch (InterruptedException ex) {
            Logger.getLogger(ImageViewTest.class.getName()).log(Level.SEVERE, null, ex);
        }
        } else {
            System.exit(0);
        }


that code should not be in your paint method, it'll block your gui (swing is single threaded)

What class are you subclassing? You probleble should be overriding paintComponent() instead of paint()

A Jlabel can actually be used to display an image, without need for subclassing.

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Author Comment

by:jsonburke
ID: 21765083
That code was from a runnable JPanel....

Could you elaborate on your thoughts of using paintComponent with a JLabel? And, what might be a better way to execute this repetition?
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Accepted Solution

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objects earned 500 total points
ID: 21765154
JLabel already supports image display via the setIcon() method, no need for subclassing.

http://java.sun.com/docs/books/tutorial/uiswing/components/label.html

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Author Comment

by:jsonburke
ID: 21765206
I will try this solution tomorrow and continue to read those docs in the mean time. Thank you for the direction thus far.
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